1. Field of the Invention
The present invention relates to a process for producing an optically active .alpha.-isopropyl-p-chlorophenylacetic acid (hereinafter referred to as ICPA) of the formula: ##STR2## wherein the symbol * stands for an asymmetric carbon atom. An optically active ICPA, in particular (+)-ICPA, is useful as an acid component for producing pyrethroids, such as fenvarelate, used as effective insecticides (cf. Japanese Patent Kokai Publication No. 136245/1980).
2. Description of the Related Art
To produce an optically active ICPA, for example, the following methods comprising optical resolution are known:
(i) an optical resolution method wherein a diastereomer salt of ICPA with an optically active amine such as .alpha.-phenyl-.beta.-(p-tolyl)-ethylamine or .alpha.-phenethylamine is subjected to crystallization (cf. Japanese Patent Kokai Publication Nos. 25544/1975 and 80627/1984), and
(ii) an optical resolution method wherein a salt of ICPA with an achiral amine such as diethylamine is subjected to preferential crystallization (cf. Agric. Biol. Chem., 46, 1421 (1982)).
In such conventional methods, a desired optically active ICPA can be obtained by decomposing the salt of the optically active ICPA with an amine after the optical resolution.
After crystallizing off the desired ICPA salt, the mother liquor is rich in the corresponding ICPA salt having the opposite optical activity. Since the optically active ICPA salt dissolved in the mother liquor is usually useless, it is industrially significant to reuse the salt by racemization and another optical resolution.
However, according to the conventional methods, the optically active ICPA salt is racemized very slowly. Therefore, in order to convert an optically active ICPA salt into the form of racemic modification, it is necessary in practice to perform a complicated process comprising decomposing the salt to give the corresponding optically active ICPA (which is the undesired ICPA enantiomer), racemizing the free ICPA, forming a salt of the obtained racemic modification with an amine, and then optically resolving the salt. In addition, the resolving agent required to carry out the method (i) is expensive and should have a high optical purity in order to obtain ICPA in a highly optically pure form.
As mentioned above, the conventional methods for production of an optically active ICPA are unsatisfactory.